Getting error while changing Depreciation Key..........

Hi,

I have two problems......

ONE: Changing the %

I wants to change the Depreciation key in asset master that is 20% Straight Line Method. Before that it was assigned a Depreciation key that is 10% SLM. But its giving the error as " Enter a depreciation key with depreciation by date"

TWO: Changing the Method

I wants to change the Depreciation key in asset master that is 20% Written down value method. Before that it was assigned a Depreciation key that is 20% WDV.

How can i do this....

Plz.. Explain me

Thanks in Advance

Abhinesh